What is your TV's model number? It sounds like your main board is damaged and your might need a replacement. Sometimes however this could be an issue with heat dispersion. I have seen a few TV's that do not work as you have described because the chips on the main board get too hot.
Does the flickering image on the screen also affect the TV's internal functions like the menu screen or volume indicator? Usually if the issue affects the internal functions you could also have an issue with the Timing Controller Board, if not its most likely the main board.